The present invention relates to imagers having discrete photosensors, and more particularly to imaging apparatus using such imagers where there may be defective photosensors.
Present CCD (charge coupled device) imagers suffer from low yields due to imperfections and defects in the integrated circuit chips from which they are fabricated. Various schemes have been used to correct for the signal disturbances that result from such defects. For example U.S. Pat. No. 3,904,818 shows a system that in effect does dropout compensation. The system detects when a photosensor provides excessive dark current, and if so, substitutes a signal that is the average of signals provided by photosensors surrounding the defective one. However, an average of signals from surrounding photosensor may not be a close enough approximation to be satisfactory. Also, other photosensors in the area may be affected by the defect and averaging may not produce an acceptable correction. Other schemes use a ROM (read only memory) programmed with correction factors for the signal from each photosensor. However, if a particular photosensor is totally defective, the signal therefrom cannot be corrected.
It is therefore an object of the present invention to correct for the effects of defective photosensors of a discrete photosensor imager.